If I get less than 6 hours, bad things happen. I'm very irritable, etc.
My record, I think, is 36 hours. If I am very sad, I might sleep an entire Saturday away.
I can take for-EVER to fall asleep. As a kid, I was always the last to be asleep at a sleepover, and then always the last to wake up. These days I deal with the insomnia a lot better. I try to avoid seeing the clock at all costs if I can help it (only raises stress), and I remind myself that it's just one night and I'll be perfectly fine. I rarely go more than a week or so with insomnia now, when 5-7 years ago I was on the verge of a nervous breakdown due to lack of sleep.
Oddly a day after I've only gotten 4 hours a sleep is typically decent. The day after that is TOUGH.